Electrode coating material and discharge device



May 3, 1966 A. w. WAINIO ETAL 3,249,738

ELECTRODE CQATING MATERIAL AND DISCHARGE DE VICE Filed Nov. 8, 1961 ALKALINE-EARTH METAL OXIDE AND CALCIUM ZIRCONATE ELECTRODE COATING 22 INVENTORS GEORGE MEISTER a ALBERT w. WAINIO United States Patent 3,249,788 ELECTRODE COATING MATERIAL AND DISCHARGE DEVICE Albert W. Wainio, Riverdale, and George Meister, Newark, N.J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 8, 1961, Ser. No. 151,067

7 Claims. (Cl. 313-109) This invention relates to discharge devices, such as fluorescent lamps and, more particularly, to electronemissive coating material for electric gaseous discharge devices and the devices which utilize such electron-emissive coating material.

The usual fluorescent lamp has electrode coils operatively disposed proximate either end of the lamp envelope and these electrode coils are coated with an electron-emissive material which is formed of a mixture of alkalineearth metal oxide. An additive of a small amount of zirconium oxide can be used in the coating material. During operation of such lamps, there is some tendency for blackening to appear at the ends of the envelope, proximate the positioning of the electrodes. Fluorescent lamps now have a rated life of 7500 hours, and it would be desirable to extend the rated life of such lamps.

It is the general object of this invent-ion to provide an improved fluorescent lamp which displays less tendency for so-called end blackening.

It is another object to provide a fluorescent lamp which has improved life.

It is a further object to provide an improved electrode coating material for electric gaseous discharge devices, in order to improve the performance characteristics of such discharge devices.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by modifying the lamp electrodes and the electron-emission coating material for such electrodes.

In accordance with the present invention,- the emission coating material comprises an alkaline-earth oxide, or preferably a mixture of alkaline-earth oxides, having added thereto from 0.6% to 9% by weight of calcium zirconate, calculated on the basis of the total alkalineearth metal oxide. 1

For a better understanding of the invention, reference should be made to the accompanying drawings wherein the sole figure is an elevational view of a fluorescent lamp, shown partly in section, which lamp incorporates the improved emission coating material of the present invention.

Patented May 3, 1966 normally made by contacts 24 which project from the supporting base caps 26 secured to either end of the elongated lamp envelope. The envelope 12 has coated on its interior surface any suitable phosphor material 28, which phosphor can be a so-called halophosphate or a manganese-activated zinc silicate, for example. As'in the case of the usual lamp, a small charge of mercury 30 and an inert, ionizable starting gas are also included in the envelope 12.

The improved electron-emission coating material of the present invention comprises a single alkaline-earth oxide, or preferably a mixture of alkaline-earth oxides, which have added thereto from 0.6% to 9% by weight of calcium zirconate (CaZrO as calculated on the weight of the total alkaline-earth oxide. As a specific example, the electron-emission coating comprises a mixture of about 60% by weight of barium oxide, about 30% by Weight of strontium oxide, and about 10% by weight of calcium oxide, and with an additive of 5% by weight of calcium zirconate evenly mixed therewith. As a specific example for preparing the present emission mixture, alkaline-earth metal carbonates are first mixed together in finely divided form in such proportions that on conversion to the oxides, the weight ratio of BaO:SrO:CaO will be 60:30:10. To these carbonates are added approximately 6% byweight of finely divided calcium zirconate. The state of division of the carbonates and zirconate is not critical, but as an example, the carbonates can have an average particle size of approximately 6 microns and the zirconate can have an average particle size of approximately 0.5 to 3 microns.

It should be understood that therelative proportion of the alkaline-earth metal oxides in the coating material can be varied considerably, as in the case of an electronemissive coating material which has a zirconia additive. Also, only one alkaline-earth metal oxide need be used, such as barium oxide, although the mixture of oxides is preferred.

In order to apply the electrode coating mixture to the filamentary coils 20, the alkaline-earth carbonates and calcium zirconate are thoroughly mixed with a solution of cup. The mixture is thoroughly milled and the electrode While the present electrode coating material has utility with respect toany electric gaseous discharge device, the material has particular utility with respect to fluorescent lampsfand hence its use in such lamps has been illustrated and will beso described.

With specific reference to the form of the invention illustrated in the drawing, the numeral 10 in FIG. 1 illustrates generally a 40W TlZ-type fluorescent lamp comprising a tubular vitreous envelope 12, which is normally fabricated of the usual soda-lime-silica glass, having mounts 14 sealed into either end thereof, as is customary. Each mount comprises a vitreous portion 16 sealed to the ends of the envelope 12 with lead-in conductors 18 sealed therethrough and supporting at their inwardly extending extremities refractory metal coils 20, which are fabricated of tungsten, for example. These coils are normally of a so-called coiled-coil construction or of a triple-coil construction, and such constructions are well known in the art. Contained within the turns of the inner coil or coils is a filling of the electron-emission material 22, as will be described in detail hereinafter.

Electrical connection for the lead-in conductors 18 is coils 20 to be coated are dipped into the emission material coating mixture. A representative amount of coated emission material is 4mg. per electrode. for a 40. watt fluorescent lamp and this amount is subject to considerable variation.

After filament coating, the mounts are sealed into the ends of the envelope 12, as illustrated in the drawing, and the electrode coils 20 are connected in series with a variable potential for purposes of treating. In such treating, the envelope 12 is first exhausted and the treating circuit energized by passing a current of approximately 300 milliamps through the coils 20L This treating current is gradually increased over a period of about 45 to 50 seconds, up to a maximum treating current of approximately 535 milliamps. This breaks down the alkaline-earth carbonates to the oxide form and the residual carbon dioxide gas is pumped from the envelope 12. Thereafter the lamps 10 are charged with the inert, ionizable starting gas .and dosed with mercury in conventional fashion.

In.the following Table I are summarized the test results obtained with a large number of lamps and also the performance of control lamps which were otherwise similar, but which had 1% by weight of zirconia additive, instead of the calcium zirconate additive of the present invention.

Table I.-Percent clean lamps (Hours operation) Calcium Zirconate Additive (percent The optimum amount of calcium zirconate additive is about 5% by weight of the alkaline-earth oxides. As shown in the foregoing table, there is considerably less blackening encountered after operation through 2,000 hours. The same degree of improvement continues throughout the life of the lamps.

The life of the lamps which incorporates the present electrode coating is also improved and an average life of from 9,000 to more than 10,000 hours is obtained from lamps incorporating the emission material of the present invention. With less than about 3% by weight of calcium zirconate additive, there is some tendency for greater blackening, but even with only 0.6% by weight of calcium zirconate additive, an improved life can be obtained.

Small amounts of impurities may be introduced into the present coating material and these can be tolerated. For example, during the electrode treating, some tungstates may be formed through interaction between the alkaline-earth metal carbonate-oxide and the coils 20.

It will be recognized that the objects of the invention have been achieved by providing an improved electric gaseous discharge device which has less tendency for blackening at the ends of the lamp proximate the electrodes. In addition, there has been provided a fluorescent lamp which has improved life. There has also been provided an emission materialfor use with fluorescent lamps.

While best examples of the present invention have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

We claim as our invention:

1. An electron-emissive coating material for electric gaseous discharge device electrodes, said coating material consisting essentially of alkaline-earth metal oxide having evenly mixed therewith from 0.6% to 9% by weight of calcium zirconate.

2. An electron-emissive coating material for electric gaseous discharge device electrodes, said coating material consisting essentially of alkaline-earth metal oxide having evenly mixed therein about 5% by weight of calcium zirconate.

3. An electron-emissive coating material for electric gaseous discharge device electrodes, said coating material consisting essentially of about 60% by Weight BaO, about 30% by weight SrO and about 10% by weight CaO, and having evenly mixed therein about 5% by weight of calcium zirconate, as calculated on the weight of total alkaline-earth metal oxide.

4. A fluorescent low-pressure discharge lamp comprising, a sealed light-transmitting elongated envelope, a phosphor coating on the inner surface of said envelope, a charge of vaporizable metal and an inert ionizable gas sealed within said envelope, filamentary electrode coils operatively disposed proximate the ends of said envelope, and a predetermined amount of electron-emissive coating material carried on said electrode coils, said coating material consisting essentially of a mixture of alkaline-earth oxide in predetermined proportions having evenly mixed therewith from 0.6% to 9% by weight of calcium zirconate.

5. A fluorescent low-pressure discharge lamp comprising, a sealed light-transmitting elongated envelope, a phosphor coating on the inner surface of said envelope, 2. charge of vaporizable metal and an inert ionizable gas sealed Within said envelope, filamentary electrode coils operatively disposed proximate the ends of said envelope, and a predetermined amount of electron-emissive coating material carried on said electrode coils, said coating material consisting essentially of a mixture of alkaline-earth oxide in predetermined proportions having evenly mixed therein about 5% by weight of calcium zirconate.

6. A fluorescent low-pressure discharge lamp comprising, a sealed light-transmitting elongated envelope, a phosphor coating on the inner surface of said envelope, a charge of vaporizable metal and an inert ionizable gas sealed within said envelope, filamentary electrode coils operatively disposed proximate the ends of said envelope, and a predetermined amount of electron-emissive coating material carried on said electrode coils, 'said coating material consisting essentially of a mixture of about 60% by weight BaO, 30% by weight SrO and 10% by weight CaO, and having evenly mixed therein about 5% by weight of calcium zirconate, as calculated on the weight of total alkaline-earth metal oxide.

7. The method of preparing an electrode for a fluorescent low-pressure discharge lamp, in order to improve the performance characteristics of such lamp, which method comprises,

(a) evenly mixing with predetermined proportions of selected finely divided alkaline-earth metal carbonates, as required to form anuntreated emission material, from 0.6% to 9% by weight of finely divided calcium zirconate, with the weight percentage of evenly mixed calcium zirconate calculated with respect to the weight of equivalent alkaline-earth metal oxide as included in the untreated emission material,

(b) coating the evenly mixed finely divided untreated emission material onto a fluorescent lamp electrode coil, and

(c) heating the electrode coil to convert the alkalineearth metal carbonates to the oxides.

References Cited by the Examiner UNITED STATES PATENTS 2,128,270 8/1938 Spanner et al. 313-211X 2,687,489 8/1954 Anderson et al. 313-211 2,806,970 9/1957 Meiser et al 313346 X GEORGE N. WESTBY, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

4. A FLUORESCENT LOW-PRESSURE DISCHARGE LAMP COMPRISING, A SEALED LIGHT-TRANSMITTING ELONGATED ENVELOPE, A PHOSPHOR COATING ON THE INNER SURFACE OF SAID ENVELOPE, A CHARGE OF VAPORIZABLE METAL AND AN INERT IONIZABLE GAS SEALED WITHIN SAID ENVELOPE, FILAMENTARY ELECTRODE COILS OPERATIVELY DISPOSED PROXIMATE THE ENDS OF THE SAID ENVELOPE, AND A PREDETERMINED AMOUNT OF ELECTRON-EMISSIVE COATING MATERIAL CARRIED ON SAID ELECTRODE COILS, SAID COATING MA- 